donderdag 24 mei 2018

Replacing a Bafang Freewheel.

The Bafang motor our velomobile motor with PAS sensor is based on, is a charming motor but the later generation has one weak point: The freewheel separating the Human Powered transmission from the Electric, is a traditional ratchet / pawl system. Unlike the velomobile motor we developed for the first hybrid velomobiles, as well as the new torque-sensor motor, both of which have unidirectional bearings, it's possible to break such a freewheel with the right amount of wrong pedalling. Not that anything is wrong with a freewheel as such. It just suffers more abuse in a crankset than in a rear wheel.

Powerful and/or unsubtle riders can break the (brittle) hardened steel pawls if they tend to kick on the pedals rather than gradually ramping up the power - not allowing the pawls to engage well before exerting maximum power.
Joint damage is somebody elses problem later on :-) but regarding the freewheels we have had - give or take - 15, or about 10% of the eWAWs we built with Bafangs, come in with broken pawls. Fortunately it only happens once after a litte education.
Since the introduction of our new velomobile motor the problem has all but gone as we tend to prescribe our new motor with clutch bearing to stronger / less experienced cyclists. We haven''t had it on other machines either, simply because the eWAWs are ridden much harder than any other velomobile or trike.

We can quickly exchange the freewheels and usually do that for free (once!), but for our distant customers and many other Bafang motor owners I thought it would be useful to write up a little manual on how to replace the freewheel but also to better understand how to treat this fine little motor.

1) First instruction is: avoiding breaking it. Remember bike motors were designed by middle-aged men with even older customers in mind :-). Let the motor do its job and take care of scorching accelerations, RESPECT THE JOINTS (to paraphrase Magnolia) and do what a human is good at: exert moderate effort over longer periods. You can push a Bafang really hard but don't KICK it. Let the pawls engage over half a turn and then push as hard as you like.

2) If you need to exchange the freewheel anyway, first remove the crank arms with an 8mm Allen key.

click the images to enlarge...

3) unscrew the plastic cap on the right hand side with a Phillips (cross) screwdriver.

4)  Not necessary for the freewheel removal but always wise: check the tightness of the large nuts. You don't have to have a dedicated tool, hammering on an old screwdriver will do just as well.

5) Pry the rubber cap on the left side loose with a small screwdriver.


 6) Unscrew the nut around the pedal axle with a pair of sharp plyers or any other couple of pointy tools. There's a second counternut just behind it, remove that one as well. Then a ring, and an axial bearing.

 A nice trick I learned from our Master Mechanic Ben is to turn the right crank to unscrew the nuts quickly.
 If all went well, this is the sequence you'd end up with when you're done with the left hand part:

 7) Remove the crank axle from the right side, and fix it in a vise.


 8) Remove the spring lockring and remove the cap


9) Unscrew the five screws in the cap holding the freewheel - we're getting there!

10) Now you can lift the axle out and we're at the freewheel at last.

You'll find that the top of some, or all of the pawls has broken off. Just replace the whole damn thing. Check the springs, don't use grease too thick and sticky: free the pawls!

 In some cases, but rarely, the outer side had worn as well. This can of course be replaced as well.

9') Now reassemble, and don't forget to put loctite on the screws.


0') After reassemby, do some test riding, then tighten the crank screws again. And again after a few hundred km.